When molded into articles such as film, copolymers of ethylene and .alpha.-olefins of 3 to 20 carbon atoms are found to have excellent mechanical strength such as tensile strength, tear strength or impact strength and also excellent heat resistance, stress crack resistance, optical characteristics and heat-sealing properties in comparison with conventional high-pressure low density polyethylenes, and are known as materials particularly useful for the preparation of inflation film or the like.
Generally speaking, the ethylene copolymers mentioned above have such excellent characteristics that when said copolymers come to be narrower in molecular weight distribution represented by the ratio (Mw/Mn) of weight average molecular weight (Mw) to number average molecular weight (Mn), the molded articles obtained therefrom, such as film, are found to be less tacky. However, when these ethylene copolymers having a narrow molecular weight distribution are melted, there were such drawbacks that their flowability represented by the ratio (MFR.sub.10 /MFR.sub.2) of MFR.sub.10 under a load of 10 kg to MFR.sub.2 under a load of 2.16 kg as measured at 190.degree. C. is small, with the result that they become poor in moldability.
Therefore, if ethylene copolymers which are small in value of Mw/Mn and narrow in molecular weight distribution and, moreover, large in value of MFR.sub.10 /MFR.sub.2 and excellent in flowability come to be obtained, such ethylene copolymers are certainly of great commercial value.
On the other hand, polypropylene has wide applications in the field of plastics because of its excellent physical properties. For example, polypropylene is widely used as packaging film material. In the applications of the type, however, because of its relatively high melting point, polypropylene is generally copolymerized with ethylene or .alpha.-olefins of 4 to 20 carbon atoms in order to improve heat-sealing properties at low temperature, and is used in the form of propylene/.alpha.-olefin copolymer.
Packaging films formed from these known propylene/.alpha.-olefin copolymers are still not sufficient in heat-sealing properties, though they are excellent in transparency and scratch resistance in comparison with those formed from low density polyethylene, and accordingly it is hoped that propylene/.alpha.-olefin copolymers excellent in heat-sealing properties even at lower temperatures will come to be obtained.
It is well known that the above-mentioned propylene/.alpha.-olefin random copolymers may be improved in heat-sealing properties by increasing the proportion of ethylene or .alpha.-olefin of 4 to 20 carbon atoms to propylene in the copolymer, However, if the proportion of ethylene or .alpha.-olefin of 4 to 20 carbon atoms is increased in the copolymerization, the resulting propylene/.alpha.-olefin copolymer increases in amount of the solvent-soluble component, whereby the resultant copolymers come to be poor in anti-blocking properties and also in stiffness.
Such propylene/.alpha.-olefin random copolymers excellent in heat-sealing properties, anti-block properties and stiffness as mentioned above are available only when they have a low melting point in spite of the fact that the proportion of .alpha.-olefin in the copolymer is small.
Incidentally, olefin polymerization catalysts composed generally of titanium compounds or vanadium compounds and organoaluminum compounds have heretofore been used for preparing ethylene copolymers. In recent years, however, catalysts composed of zirconium compounds and aluminoxane have been proposed of late as new Ziegler polymerization catalysts. Japanese Patent L-O-P Publn. No. 19309/1983 discloses a process for polymerizing ethylene and one or two more C.sub.3 -C.sub.12 .alpha.-olefins at a temperature of from -50.degree. C. to 200.degree. C. in the presence of a catalyst composed of a transition metal containing compound represented by the following formula EQU (Cyclopentadienyl).sub.2 MeRHal
wherein R is cyclopentadienyl, C.sub.1 -C.sub.6 alkyl or halogen, Me is a transition metal, and Hal is halogen, and a linear aluminoxane represented by the following formula EQU Al.sub.2 OR.sub.4 (Al (R)--O).sub.n
wherein R is methyl or ethyl, and n is a number of 4-20, or a cyclic aluminoxane represented by the following formula ##STR1## wherein R and n are as defined above. The publication cited above describes that ethylene should be polymerized in the presence of small amounts, up to 10% by weight, of somewhat long chain .alpha.-olefins or mixtures thereof in order to control a density of the resulting polyethylene.
Japanese Patent L-O-P Publn. No. 95292/1984 discloses an invention relating to a process for preparing a linear aluminoxane represented by the following formula ##STR2## wherein n is 2-40, and R is C.sub.1 -C.sub.6 alkyl, and cyclic aluminoxane represented by the following formula, ##STR3## wherein n and R are as defined above. The publication cited above describes that olefins are polymerized in the presence of the aluminoxane prepared by the process of said publication, for example, methyl aluminoxane in admixture with a bis(cyclopentadienyl) compound of titanium or zirconium, whereupon at least twenty-five million g of polyethylene per 1 g of the transition metal per hour is obtained.
Japanese Patent L-O-P Publn. No. 35005/1985 discloses a process for preparing olefin polymerization catalysts, wherein an aluminoxane compound represented by the following formula ##STR4## wherein R.sup.1 is C.sub.1 -C.sub.10 alkyl, and R.sup.0 is R.sup.1 or represents -O- by linkage therewith, is first allowed to react with a magnesium compound, the resulting reaction product is then chlorinated, followed by treating with a Ti, V, Zr or Cr compound. This publication cited above describes that the catalysts prepared by the process of said publication are particularly useful for the copolymerization of mixtures of ethylene and C.sub.3 -C.sub.2 .alpha.-olefins.
Japanese Patent L-O-P Publn. No. 35006/1985 discloses a combination of (a) mono-, di- or tri-cyclopentadienyl of two or more different transition metals or derivatives thereof and (b) aluminoxane as a catalyst system for preparing reactor blend polymers. Example 1 of the above-cited publication discloses that polyethylene having a number average molecular weight of 15,300, a weight average molecular weight of 36,400 and containing 3.4% of the propylene component has been obtained by polymerization of ethylene with propylene in the presence of a combination of bis(pentamethylcyclopentadienyl)zirconium dimethyl and aluminoxane as the catalyst. Example 2 of the said publication discloses polymerization of ethylene with propylene in the presence of a combination of bis(pentamethylcyclopentadienyl)zirconium dichloride, bis(methylcyclopentadienyl)zirconium dichloride and aluminoxane as the catalyst, whereby a blend of polyethylene and ethylene/propylene copolymer is obtained, said polyethylene having a number average molecular weight of 2,000, a weight average molecular weight of 8,300 and the propylene content of 7.1 mol%, and comprising a toluene-soluble portion having a number average molecular weight of 2,200, a weight average molecular weight of 11,900 and the propylene content of 30 mol% and a toluene-insoluble portion having a number average molecular weight of 3,000, a weight average molecular weight of 7,400 and the propylene content of 4.8 mol%. Similarly, Example 3 of the said publication discloses a blend of LLDP and an ethylene/propylene copolymer, said LLDPE comprising a soluble portion having a molecular weight distribution (Mw/Mn) of 4.57 and the propylene content of 20.6% and an insoluble portion having the molecular weight distribution of 3.04 and the propylene content of 2.9 mol%.
Japanese Patent L-O-P Publn. No. 35007/1985 discloses a process for polymerizing ethylene alone or together with .alpha.-olefins of at least 3 carbon atoms in the presence of a catalyst system containing metallocene and a cyclic aluminoxane represented by the following formula ##STR5## wherein R is alkyl of 1-5 carbon atoms, and n is an integer of 1 to about 20, or a linear aluminoxane represented by the following formula ##STR6## wherein R and n are as defined above. According to the said publication, the polymers obtained by the above-mentioned process are alleged to have a weight average molecular weight of from about 500 to about 1,400,000 and a molecular weight distribution of 1.5-4.0.
Japanese Patent L-O-P Publn. No. 35008/1985 discloses polyethylene or copolymers of ethylene and C.sub.3 -C.sub.10 .alpha.-olefins, both having a broad molecular weight distribution, obtained by using a catalyst system containing at least two kinds of metallocenes and aluminoxane. The said copolymers disclosed in the above-mentioned publication are alleged to have a molecular weight distribution (Mw/Mn) of 2-50.
Japanese Patent L-O-P Publn. No. 130314/1986 discloses polypropylene high in isotacticity obtained by polymerization of propylene in the presence of a catalyst system comprising a sterically fixed zirconium-chelate compound and aluminoxane.
J. Am. Chem. Soc., 109, 6544 (1987) discloses formation of a high molecular weight isotactic polypropylene obtained by polymerization of propylene in the presence of a catalyst system comprising ethylenebis(indenyl)hafnium dichloride or its hydride and aluminoxane, said isotactic polypropylene having a narrow molecular weight (Mw/Mn) of 2.1-2.4.
Japanese Patent L-O-P Publn. No. 142005/1987 discloses a stereoblock polypropylene having Mw/Mn of 5.0-14.9 obtained by polymerization of propylene in the presence of a catalyst system comprising tetramethylethylenebis(cyclopentadienyl) titanium chloride and aluminoxane. The polypropylene thus obtained is short in isotactic chain length and is a rubbery polymer.
The present inventors have come to accomplish the present invention on the basis of their finding that ethylene copolymers which are small in Mw/Mn and narrow in molecular weight distribution and, moreover, large in MFR.sub.10 /MFR.sub.2 and excellent in flowability are obtained by copolymerization of ethylene with .alpha.-olefins of 3-20 carbon atoms in the presence of olefin polymerization catalysts composed of specific hafnium compounds and organoaluminum oxy-compounds.
Furthermore, the present inventors have found that when propylene and .alpha.-olefins of 4-20 carbon atoms are copolymerized in the presence of olefin polymerization catalysts composed of specific hafnium compounds and aluminoxane, there are obtained propylene/.alpha.-olefin copolymers which are narrow in molecular weight distribution and small in amount of the .alpha.-olefin copolymerized therewith, but are low in melting point in comparison with conventionally known propylene/.alpha.-olefin copolymers, on which the present invention has been based.
Accordingly, the present invention is to solve such problems associated with the prior art as mentioned above, and an object of the invention is to provide ethylene copolymers which are small in Mw/Mn and narrow in molecular weight distribution and, moreover, which are large in MFR.sub.10 /MFR.sub.2 and excellent in flowability, and processes for preparing the same.
A further object of the invention is to provide propylene/.alpha.-olefin copolymers which are narrow in molecular weight distribution and small in amount of the .alpha.-olefin copolymerized therewith but have a low melting point and, moreover, which are excellent in heat-sealing properties and also excellent in anti-block properties and stiffness, and processes for preparing the same.
Another object of the invention is to provide olefin polymerization catalysts, which produce polymers which are narrow in molecular weight distribution in homopolymerization or narrow in molecular weight and composition distribution in copolymerization with high polymerization activities even by the use of small amounts of aluminoxane, furthermore, produce easily polymers which are high in molecular weight.